There are known inkjet printers that print texts and patterns by ejecting liquid ink onto a recording medium such as a sheet and a cloth. Such inkjet printers are capable of outputting a two-dimensional image onto a recording medium such as a sheet, cloth, or the like, by controlling ink ejection while moving the inkjet head having a plurality of channels (ink ejector), relative to the recording medium. The ink ejection can be performed by using an actuator (a piezoelectric actuator, an electrostatic actuator, a thermal actuator, or the like), or by generating air bubbles on the ink within a tube by heat. In particular, piezoelectric actuators have recently been widely used for their advantages of large output, modulability, high responsiveness, adaptability to any type of ink, or the like.
The piezoelectric actuator is classified into two types: one using a bulk-state piezoelectric body and the other using a thin film piezoelectric body (piezoelectric thin film). The former type has a large output enough to eject large ink droplets, but unfortunately, this type is large-sized and thus is high in cost. In contrast, the latter type has a small output and thus is not capable of increasing the amount of droplet, but is compact and thus is low in cost. It is, therefore, reasonable to consider that forming an actuator with a piezoelectric thin film is suitable to achieve compact and high-resolution printers (small ink droplets would be allowable) at low cost. Note that whether to use the piezoelectric thin film or the bulk-state piezoelectric body on a piezoelectric actuator may be selected according to usage. It is possible to selectively use bulk-state piezoelectric body or thin-film type according to the print image size, the print speed, the equipment size, or the like.
FIG. 18A is a plan view illustrating a schematic configuration of a known inkjet head 100 using a piezoelectric actuator. FIG. 18B is a cross-sectional view taken along line D-D′ in the plan view. FIG. 18C is cross-sectional view taken along line E-E′ in the cross-sectional view. For the reason of convenience, illustrations of a lower electrode 201 and an upper electrode 203 of a drive element 104 described below are omitted in the plan view. The inkjet head 100 is configured such that a support substrate 101 including a plurality of pressure chambers 101a is sandwiched between a vibration plate 102 and a nozzle plate 103, and a drive element 104 including a piezoelectric body is formed on the vibration plate 102 above each of the pressure chambers 101a. The nozzle plate 103 includes a nozzle hole 103a to eject ink inside each of the pressure chambers 101a to the outside.
In addition to the plurality of pressure chambers 101a, two ink flow paths 101b are arranged in parallel on the support substrate 101. The plurality of pressure chambers 101a is formed in zigzag in two rows between the two ink flow paths 101b, 101b. The pressure chamber 101a in one row communicates with one ink flow path 101b via a communication passage 101c (ink stop), the pressure chamber 101a in the other row communicates with the other ink flow path 101b via another communication passage 101c. Moreover, one end of each of the ink flow paths 101b communicates with an ink container (ink storage tank) (not illustrated) via an ink supply port 105, while the other end communicates with the ink container via an ink discharge port 106.
The recording medium such as a sheet and a cloth moves relatively in the up-down direction of the sheet surface in the plan view in FIG. 18A. For a given constant moving speed of the recording medium, the resolution in the up-down direction is determined by the amount of droplet and the drive frequency of the channel, and the resolution in the left-right direction is determined by the amount of droplet and a channel pitch (p). In order to achieve high-resolution drawing, reduction of the channel pitch would be needed. On the other hand, in order to achieve a desired amount of droplet, a constant area for the pressure chamber (size of the pressure chamber) would be needed. In order to cope with this spatial incompatibility therebetween, there is a technique of providing a multiple rows of channels in the up-down direction so as to reduce an apparent pitch.
In order to allow the inkjet head 100 to achieve printing machine-quality high resolution as high as about 600 dpi to 2400 dpi (dot per inch), several to several tens of rows would be needed. Increasing the number of rows, however, would also increase the head area. An enlarged head would increase the size and cost of the apparatus, and in addition, image quality would be deteriorated due to speed variation within a row or misalignment between the head and the recording medium.
In order to eliminate this inconvenience, there is a need to arrange the channels with a predetermined size in high density. To achieve high-density arrangement of the channels, it would be desirable to use flexible arrangement in which the directions of the channels (herein, corresponds to a direction from the ink flow path 101b to the pressure chambers 101a) are alternated, as illustrated in FIG. 18C.
Next, channels of the known inkjet head 100 will be described in detail. FIG. 19A is a plan view of one channel of the inkjet head 100. FIG. 19B is a cross-sectional view taken along line F-F′ in the plan view. The drive element 104 is formed on the vibration plate 102 via an insulating layer 107. The drive element 104 is formed with the lower electrode 201, a piezoelectric body 202, and the upper electrode 203 stacked in the order from the vibration plate 102 side. The lower electrode 201 is an electrode shared by all the drive elements 104. The upper electrode 203 is separately connected with wiring unit 301 via a lead-out unit 301a having a small width. The lead-out unit 301a and the wiring unit 301 are formed on the piezoelectric body 202 drawn from above the pressure chambers 101a along the lower electrode 201. The lower electrode 201 and the wiring unit 301 are electrically connected with a drive circuit 108 via electrical wiring.
When voltage is applied from the drive circuit 108 to the lower electrode 201 and the upper electrode 203, the piezoelectric body 202 is stretched in a direction perpendicular to the thickness direction. Subsequently, the difference in length between the piezoelectric body 202 and the vibration plate 102 generates curvature on the vibration plate 102, leading to the occurrence of displacement (curve) of the vibration plate 102 in the thickness direction. The displacement of the vibration plate 102 gives pressure to the inside of the pressure chambers 101a, thereby ejecting the ink inside the pressure chambers 101a to the outside as droplets via the nozzle hole 103a. Hereinafter, note that, the vibration plate 102, the insulating layer 107, and the drive element 104, located above the pressure chambers 101a, will be collectively referred to as an actuator 110.
As the piezoelectric body 202 used for the drive element 104, perovskite-type metal oxide such as barium titanate (BaTiO3) and lead zirconate titanate (Pb(Ti/Zr)O3) referred to as PZT is widely used. For constituting a piezoelectric body using a piezoelectric thin film, PZT is formed on the substrate, for example, by deposition. Deposition of the PZT can be performed by various techniques including sputtering, a chemical vapor deposition (CVD) method, and a sol-gel method. Note that crystallization of piezoelectric materials needs high temperature, and therefore, silicon (Si) is often used as the substrate. In the case of using the bulk-state piezoelectric body separately produced by a firing method, it would be allowable to fix this piezoelectric body onto the substrate using bonding or screwing.
Meanwhile, the support substrate 101 beneath the lead-out unit 301a includes a recess (opening) 101d having a width smaller than the width of the pressure chambers 101a. This is due to the following reason. That is, since the piezoelectric body 202 and the lower electrode 201 exist beneath the lead-out unit 301a, the piezoelectric body 202 sandwiched therebetween is stretched when voltage is applied to the lead-out unit 301a and the lower electrode 201. Under this situation, in a case where the recess 101d is not formed on the support substrate 101, the vibration plate 102 located between the lead-out unit 301a and the support substrate 101 would not substantially be deformed (vibrated) when the voltage is applied Accordingly, stress might be concentrated onto the small-width lead-out unit 301a and the piezoelectric body 202 existing beneath (in particular, around a boundary with the pressure chambers 101a), and thus, might damage the piezoelectric body 202 and the lead-out unit 301a. By providing the recess 101d beneath the lead-out unit 301a of the support substrate 101, however, the vibration plate 102 on the recess 101d would be deformed when voltage is applied, making it possible to disperse the stress applied to the lead-out unit 301a and to prevent damage on the lead-out unit 301a. The configuration that includes a recess (sub-chamber, buffer chamber) on the substrate beneath the lead-out unit 301a in order to prevent the damage on the lead-out unit 301a is also disclosed in Patent Literature 1, for example.
Note that, hereinafter, the pressure chambers 101a and the recess 101d will collectively be referred to as a pressure chamber P, for the reason of convenience. In this case, the pressure chamber P can be considered to have a rotationally asymmetric shape on a plan view (viewed from actuator 110 side) as illustrated in the plan view in FIG. 19A. Moreover, hereinafter, a direction from the pressure chambers 101a toward the recess 101d will be referred to as a direction of the pressure chamber P.
Meanwhile, the actuator 110 and the pressure chamber P are individually processed with a photoengraving (photolithography) technique. Since this technique uses a mask processed in high accuracy, misalignment would not easily occur in processing on a same surface. However, since the actuator 110 and the pressure chamber P are formed by processing from mutually opposite sides with respect to the support substrate 101, it would be difficult to form the both with a same step. Accordingly, they need to be formed in separate steps. Front and rear sides of a substrate normally have reference marks. In a case where different surfaces of the substrate are going to be processed, alignment is ordinarily performed by visually checking both marks. However, since the substrate (e.g., silicon substrate) is opaque, the substrate needs to be processed while images viewed from individual front and rear surfaces of the substrate are aligned. This processing tends to induce misalignment.
When the actuator 110 is located on the pressure chamber P having a rotationally asymmetric shape in a plan view, planar misalignment possibly occurring between the pressure chamber P and the actuator 110 might lead to a problem of ink ejection characteristic (injection performance) variation attributed to the misalignment.
FIG. 20 schematically illustrates a positional relationship of the actuator 110 with respect to the pressure chamber P having a rotationally asymmetric shape in plan view. In the diagram, the directions of the pressure chambers P are different between in upper-row channels (channels in (2) (4)) and in lower-row channels (channels in (1) (3)) so as to arrange the channels in high density. In the diagram, with two directions perpendicular to each other within a plane parallel to the support substrate 101 being defined as an X-direction and a Y-direction, pattern 1 illustrates a case where the actuator 110 has no misalignment with respect to the pressure chamber P, and patterns 2 to 4 respectively illustrate cases where the actuator 110 is misaligned in the Y-direction, X-direction, and both X- and Y-directions with respect to the pressure chamber P.
In pattern 2, the actuator 110 is close to a portion with no recess (pressure chamber P) on the support substrate 101 in plan view, on the lower-row channels. The portion with no recess on the support substrate 101 has high rigidity. Therefore, even when the piezoelectric body 202 of the actuator 110 is stretched rightward and leftward (in direction parallel to substrate) at driving, the vibration plate 102 is not easily deformed in a direction perpendicular to the substrate. In this case, since displacement of the vibration plate 102 is reduced, the pressure transmitted to the ink inside the pressure chamber P is decreased, leading to the decrease in injection speed and the amount of droplet. Meanwhile, the actuator 110 is misaligned in the Y-direction also in the upper-row; channels. However, since there is the recess 101d as a buffer chamber of the pressure chamber P, rigidity is low and displacement of the vibration plate 102 would not be reduced so much.
In pattern 3, the actuators 110 in all channels are misaligned in the X-direction. Accordingly, similarly to the lower-row of pattern 2, displacement of the vibration plate 102 is reduced in all the channels.
Displacement reduction of the vibration plate 102 in pattern 4 corresponds to a combination of patterns 2 and 3. The displacement reduction of the vibration plate 102 is greater in the lower-row channels rather than in the upper-row channels.
The recording medium such as a sheet and a cloth moves relatively in the Y-direction in FIG. 20, with respect to the head, for example. In examples in patterns 2 and 4, ejection performance deterioration is greater in the lower-row channels rather than in the upper-row; channels. As a result, streaky gradation unevenness is generated on the recording medium.
Note that, in a case where ejection performance changes evenly across all channels as in pattern 3, it is possible to manage this issue by uniformly adjusting drive circuit output. Unfortunately, however, since almost all cases like patterns 2 and 4 include misalignment in the Y-direction of relative movement of the recording medium, uniform adjustment for all the channels cannot handle the problem.
In this manner, forming the pressure chamber P and the actuator 110 with different steps onto the front-back of the substrate would generate a difference in output due to misalignment at processing, and image quality deterioration caused by this difference.
In this regard, Patent Literatures 2 to 4 disclose, for example, a technique of correcting image gradation by adjusting drive signals in accordance with the magnitude of ejection performance with independent amplifiers, resistors, and correction memories, provided for individual channels. These techniques, however, need to arrange elements for individual channels, leading to an increase in cost and the head size.
Meanwhile, there is a technique, similar to the above-described technique, of controlling the amount of ink ejection for individual channel rows. For example, Patent Literatures 5 and 6 describe a technique to suppress, on the head including a plurality of rows of channels, image quality deterioration with uneven density by controlling the amount of injection for the individual rows. Moreover, Patent Literature 7 describe a technique to suppress image deterioration induced by uneven temperature, by arranging a functional circuit configured to control constant current drive of individual switching elements, on a head configured to eject ink by performing constant current drive of a plurality of heaters arranged in the row direction, using individual switching elements.